1. Field of the Invention
The present invention relates to a loudspeaker using an ultrasonic wave and more particularly, to a super-directional loudspeaker having electro-acoustic transducer elements arranged on a curved surface to converge on a point, which makes it possible for a listener to listen anytime a sound emitted from the loudspeaker at a high sound pressure.
2. Description of the Prior Art
Conventionally, it has been known that a loudspeaker system with high directivity can be realized by using an ultrasonic wave.
For example, the Japanese Non-Examined Patent Publication No. 3-159400 published in July 1991 discloses a super-directional loudspeaker system comprising a super-directional loudspeaker using a parametric array, an ultrasonic wave receiver for receiving an ultrasonic wave beam which is emitted from the loudspeaker and reflected by a listener, and a controller for selecting sound sources on the basis of the reflected ultrasonic wave beam received by the receiver and for applying the selected sound source to the loudspeaker.
A first input signal, which is produced in a first one of the sound sources selected by the controller, is modulated and amplified to produce a first output signal. The first output signal is then applied to the super-directional loudspeaker, thereby emitting an ultrasonic wave beam. At this time, a first audible sound (e.g., a background music) according to the first input signal is emitted from the loudspeaker along with the ultrasonic wave beam thus emitted.
If a listener exists at a position in front of the loudspeaker, the listener can listen to the emitted first sound and at the same time, a part of the emitted ultrasonic wave beam is reflected by the listener and received by the ultrasonic wave receiver. If the level of the received ultrasonic wave beam is greater than a specific threshold value, a second one of the sound sources is selected by the controller instead of the first sound source. Then, a second input signal produced in the second sound source is modulated and amplified to produce a second output signal. The second output signal is applied to the super-directional loudspeaker, thereby emitting an ultrasonic wave beam containing a second audible sound (e.g., a shopping information) according to the second input signal. In this case, the listener listens to the emitted second sound.
If the level of the reflected ultrasonic wave beam is equal to or less than the specific threshold value or no reflected ultrasonic wave beam exists, the first sound is kept being emitted and the listener keeps listening to the first sound.
As described above, in the conventional super-directional loudspeaker system disclosed in the Japanese Non-Examined Patent Publication No. 3-159400, an ultrasonic wave beam is used as a carrier for an audio input signal of an audio frequency. Specifically, a high-frequency signal of an ultrasonic frequency is modulated by an input signal of an audio frequency. The modulated high-frequency signal is applied to electro-acoustic transducer elements of the loudspeaker, thereby emitting high-directional ultrasonic waves containing an audible sound according to the input signal. The ultrasonic waves thus emitted propagate in the air as a super-directional ultrasonic wave beam. The electro-acoustic transducer elements of the loudspeaker are arranged on a flat surface and as a result, the emitted ultrasonic waves propagate in parallel in the air.
Moreover, the Japanese Non-Examined Patent Publication No. 3-296399 published in December 1991 discloses a parametric loudspeaker system comprising a loudspeaker unit having ultrasonic oscillators arranged on a plate- or rod-shaped base, and a rotating means for rotating the loudspeaker unit around a specific rotation axis while the ultrasonic oscillators are located to face the rotation axis. The loudspeaker unit is rotatable so as to keep an angle with respect to the rotation axis acute, where the angle can be adjusted by the rotating means as necessary.
A high-frequency signal of an ultrasonic frequency is modulated by an input signal of an audio frequency and amplified. The modulated and amplified signal is then applied to the ultrasonic oscillators of the loudspeaker unit, thereby emitting high-directional ultrasonic waves containing an audible sound according to the input signal. Since the ultrasonic oscillators of the loudspeaker unit are arranged on the plate- or rod-shaped base, the emitted ultrasonic waves propagate in parallel in the air as a beam.
Further, the loudspeaker unit is rotated around the rotation axis to form a circular cone. Therefore, the ultrasonic waves emitted from the ultrasonic oscillators are converged on a point where a listener is located in front of the loudspeaker unit. If the acute angle between the loudspeaker unit and the rotation axis is changed in value by the rotating means, the focusing point of the ultrasonic waves can be changed so as to follow the change of the point of the listener.
As described above, in the conventional parametric loudspeaker system disclosed in the Japanese Non-Examined Patent Publication No. 3-296399, similar to that disclosed in the Japanese-Non Examined Patent Publication No. 3-159400, an ultrasonic wave beam is used as a carrier for an audio input signal of an audio frequency. Specifically, a high-frequency signal of an ultrasonic frequency is modulated by an input signal of an audio frequency. The modulated high-frequency signal is applied to electro-acoustic transducer elements arranged on a flat surface, thereby emitting ultrasonic waves containing an audible sound according to the input signal. However, unlike the case of the Japanese-Non Examined Patent Publication No. 3-159400, the ultrasonic waves emitted from the loudspeaker unit propagate to be focused on an optional point.
FIG. 1 is a block diagram showing the common basic configuration of the above-described two conventional loudspeaker systems.
As shown in FIG. 1, an audio signal source 110 generates an electric audible signal S101 of a variable audio frequency. A high-frequency oscillator 150 generates an electric high-frequency signal S102 of a fixed ultrasonic frequency. An amplitude modulator 120 amplitude-modulates the high-frequency signal S102 by the audio signal S101, thereby producing a modulated ultrasonic signal S103. A power amplifier 130 amplifies the modulated ultrasonic signal S103 to produce an amplified ultrasonic signal S104.
An electro-acoustic transducer unit (i.e., a loudspeaker unit) comprise a plurality of electro-acoustic transducer elements 145 arranged on a flat surface of a suitable supporting member (not shown). The transducer elements 145 convert the amplified ultrasonic signal S104 to acoustic vibrations of the same ultrasonic frequency as that of the high-frequency signal S102. The acoustic vibrations of the same ultrasonic frequency, which are produced by the transducer elements 145, generate high-directional ultrasonic waves USW and emit them into the air. The ultrasonic waves USW thus emitted propagate in the air as an ultrasonic wave beam with a super directivity.
While the ultrasonic waves USW propagate in the air, a nonlinear interaction occurs between the ultrasonic waves USW and the air, resulting in demodulation operation of the ultrasonic waves USW. As a consequence, an audible sound according to the audio signal S101 of the audio frequency is generated in the air and transferred by the beam of the ultrasonic waves USW. In other words, a super-directional audible sound wave is generated in the air. This phenomenon has been termed the xe2x80x9cparametric array effectxe2x80x9d.
If a listener is located at any one of locations in the propagation direction of the beam of the ultrasonic waves USW, the listener can listen to the audible sound. However, if the listener is located out of the propagation direction, the listener is unable to listen to the audible sound because of its super directivity.
With the conventional loudspeaker system having the conventional common basic configuration shown in FIG. 1, however, the following problems will occur.
A first one of the problems is that the listener is unable to receive the audible sound at a satisfactorily high sound pressure. This problem is caused by the fact that the electro-acoustic transducer elements 145 are arranged on the flat surface and therefore, the energy of the acoustic vibrations formed by the elements 145 is likely to scatter or diffuse in the air. In other words, the listener tends to receive only a small part of the acoustic vibrations.
A second one of the problems is that the circuit configuration of the loudspeaker system is complicated and the fabrication cost thereof is high. This problem is caused by the fact that the number of the transducer elements 145 needs to be increased in order to raise the sound pressure of the audible sound emitted by the elements 145, thereby increasing the overall output of the transducers 145. At the same time, this problem is also caused by the fact that the gain of the power amplifier 130 needs to be higher.
On the other hand, a technique to converge an acoustic vibration emitted from an electro-acoustic transducer element on a point by the use of a paraboloidal reflector is disclosed in the Technical Report of the Institute of Electronics, Information and Communication Engineers (IEICE), pp. 25-30, EP94-37, August 1994, which is entitled xe2x80x9ca Spatial Sound Source Made by Focused Parametric Array Sound Beamxe2x80x9d.
In this technique, an electro-acoustic transducer element is located in front of a paraboloidal concave surface of a paraboloidal reflector. When the transducer element emits an acoustic vibration of an ultrasonic frequency according to an applied input signal, an ultrasonic wave is emitted from the element toward the reflector at a specific solid angle. The ultrasonic wave thus emitted propagates in the air to the reflector and then, is reflected by the paraboloidal concave surface of the reflector. Thus, the reflected ultrasonic wave propagates in the air so as to converge on a point in front of the concave surface.
Therefore, the previously-described first and second problems may be solved by applying the above-described technique disclosed in the technical report of IEICE to one of the conventional loudspeaker system disclosed in the Japanese-Non Examined Patent Publication Nos. 3-159400 and 3-296399. In this case, however, a problem that the size of the loudspeaker unit of the loudspeaker system becomes large will occur. This problem is caused by the following two reasons.
Specifically, first, to converge the reflected ultrasonic wave on the converging point, the reflector having the paraboloidal concave surface is essentially located apart from the electro-acoustic transducer element by a specific distance. The technical report of IEICE discloses that an example of the distance between the paraboloidal concave surface of the reflector and the transducer element is 15 cm.
Second, the ultrasonic wave emitted from the transducer element is likely to spread three-dimensionally at a specific solid angle. Therefore, the paraboloidal concave surface of the reflector needs to be comparatively wide.
Accordingly, an object of the present invention to provide a super-directional loudspeaker that make it possible for a listener to receive an audible sound at a high sound pressure with a compact size.
Another object of the present invention to provide a super-directional loudspeaker that make it possible for a listener to receive an audible sound at a high sound pressure while preventing the circuit configuration of a loudspeaker system from being complicated and the fabrication cost thereof from being high.
Still another object of the present invention to provide a super-directional loudspeaker in which the location of a listening point is readily adjustable according to the location change of a listener.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
A super-directional loudspeaker according to the present invention is comprised of a supporting member having a concave surface, and electro-acoustic transducer elements fixed to the supporting member.
The elements are designed to receive an electrical input signal and to produce acoustic vibrations according to the electrical input signal thus received, thereby emitting directional ultrasonic waves in the air. The elements are arranged along the concave surface of the supporting member in such a way that the directional ultrasonic waves emitted by the elements propagate in the air to converge on a listening point in front of the concave surface.
With the super-directional loudspeaker according to the present invention, the electro-acoustic transducer elements are fixed to the supporting member to be arranged along the concave surface thereof. The elements are designed to receive the electrical input signal and to produce the acoustic vibrations according to the electrical input signal thus received, thereby emitting the directional ultrasonic waves in the air. Further, the elements are arranged along the concave surface of the supporting member in such a way that the directional ultrasonic waves from the elements propagate in the air to converge on the listening point in front of the concave surface.
Accordingly, if a listener is located at the listening point, he can receive an audible sound generated by the directional ultrasonic waves emitted from the elements at a high sound pressure.
Also, the electro-acoustic transducer elements are fixed to the supporting member to be arranged along its concave surface. In other words, the elements are not provided apart from supporting member. Therefore, the super-directional loudspeaker according to the present invention has a compact size and at the same time, the circuit configuration of a loudspeaker system using this loudspeaker is not complicated and the fabrication cost thereof is not high.
In a preferred embodiment of the super-directional loudspeaker according to the present invention, a curvature of the concave surface of the supporting member is adjustable. In this case, there is an additional advantage that the location of the listening point is readily adjustable according to the location change of the listener.
In another preferred embodiment of the super-directional loudspeaker according to the present invention, the electro-acoustic transducer elements are arranged regularly with respect to a center of the concave surface of the supporting member. In this case, there is an additional advantage that an obtainable sound pressure at the listening position becomes higher.
In this preferred embodiment, it is preferred that the electro-acoustic transducer elements are arranged circularly around the center of the concave surface of the supporting member, or closely adjacent to one another around the center of the concave surface of the member.
In still another preferred embodiment of the super-directional loudspeaker according to the present invention, a listener position recognizer and a curvature controller are additionally provided.
The listener position recognizor recognizes a listener position and outputs a position signal. The curvature controller controls a curvature of the concave surface of the supporting member according to the position signal from the listener position recognizor so that the listening point is overlapped with the recognized listener position. In this case, there is an additional advantage that even if the listener position is changed, the listener can always listen to the audible sound generated by the emitted directional ultrasonic waves.
In a further preferred embodiment of the super-directional loudspeaker system according to the present invention, the listener position recognizor has an acoustic-electric transducer for converting a reflected ultrasonic wave by the listener to an electric position signal, a delay time detector for detecting a delay time of the reflected ultrasonic wave from a difference between the electric position signal and the electrical input signal, thereby generating a delay time signal of the reflected ultrasonic wave, and a distance calculator for calculating a distance between the listening point and the listener position from the delay time signal. In this case, there is an additional advantage that the listener position recognizer is readily configured.
It is preferred that well-known piezoelectric transducer elements are used as the electro-acoustic transducer elements provided in the loudspeaker according to the first aspect of the present invention. This is because the piezoelectric transducer elements are capable of electro-acoustic and acousto-electric transducer operations and therefore, the piezoelectric transducer elements can be used not only as the electro-acoustic transducer elements fixed to the supporting member but also as the acousto-electric transducer element of the listener position recognizor.
The acousto-electric transducer may be fixed to the supporting member for the electro-acoustic transducer elements or provided apart from the supporting member.
It is preferred that the concave surface of the supporting member is formed by sector-shaped blades that are movable around the center of the concave surface. The blades are moved like a well-known aperture shutter of cameras by the curvature controller to change the curvature of the concave surface, thereby keeping an obtainable sound pressure at the listening point maximum.